associations between insects
and microbes (microbe/host)
• Commensalism (+/0)
• Parasitism (+/-)
• Mutualism (+/+)
all are forms of symbiosis
Discovery of Wolbachia
• In mosquito breeding experiments in 1924,
some crosses were completely sterile
• Due to presence of this bacteria
• Widely occurring in insects (20-75%
infection rate by species)
• In most, no obvious effects documented
• Some insects have >1 Wolbachia species
Female biased
sex-ratios
Infected hosts
more successful
Microbial Associates of Insects
• Wolbachia – a diversity of effects on hosts
• Bacteria produce an antibiotic for bee
wolves (example of providing a specific
product)
• Nutritional assistance: obligate mutualists
• Facultative guests
• A parade of partners
• External mutualists: ants and fungus
Wolbachia has different types
of effects on hosts
• Cause female-biased sex ratios of
offspring
• Increase host fitness by sterilizing nonhosts
• Obligate mutualist
• Benign reproductive parasite
Parthenogenesis-inducing
Parthenogenesis-Inducing
Wolbachia
Wolbachia
Encarsia wasp
• Parthenogenesis • Cytoplasmic
incompatibility
• Feminization
1
sex determination in
Hymenoptera
Uninfected females produce both males
and females
Infected females produce only females
• haplodiploid system
• unfertilized eggs (haploid) become
males
• fertilized eggs (diploid) become females
Parthenogenesis-inducing (PI)
bacteria increase in frequency in
a population by
converting males to females
How do they do that? Found only in
haplodiploid sex determinating systems Hymenoptera, Thysanoptera, some mites
Uninfected
Infected
Parthenogenesis-inducing (PI)
bacteria converting males to females
Uninfected
nn
Infected
n
n
n
n→2n
n
2n
2n
2n
Male
Feminization
• Ostrinia moth
• Males are homogametic
sex (ZZ)
Female
Female
Female
feminization
• Wolbachia infected males (ZZ) develop
as females
• When these females are treated with
antibiotics, and mate
with normal males
(ZZ) all offspring are
male
• maintained at low
level in population
2
Female
Female biased
sex-ratios
Infected hosts
more successful
• Parthenogenesis • Cytoplasmic
incompatibility
• Feminization
X
Male
Offspring
X
uninfected
X
infected
X
NOTHING =
incompatible
X
infected,
RESCUE!
What causes the incompatibility and
allows rescue?
• Best current model is “modification-rescue”
• Sperm nucleus modified by Wolbachia so that it
cannot successfully fuse with an egg nucleus
unless some product in cytoplasm of infected egg
‘rescues’ it
Wolbachia and
Onchocerca volvulus
• black flies
transmit
Onchocerciasis =
river blindness
• nematodes that
form nodules,
eventually end up
in eyes
Wolbachia as a mutalist
Wolbachia and
Onchocerca volvulus
• treatment first with
anti-nematode
chemical (ivermectin)
• recently awareness of
symbiont led to
treatment with
antibiotics
• antibiotics kills the
worms!
3
Beewolves - Philanthus
• prey on bees
• nest in sandy soil
• larvae feed on
bees
• spin cocoon,
overwinter
• moist nest – risk
of infection
Beewolves - Philanthus
sweet
bacterial home
•
•
•
•
bacterial symbionts on antennae
Streptomyces
vertically transmitted
secreted into brood cell
and taken up by larvae
and incorporated into
cocoon*
• each species has its own
species of Streptomyces
• abt 26 myo
Many insects have extreme
diets - they are very
unbalanced in nutrients –
lacking proteins, vitamins, etc
The Aphid and Buchnera
sap, wood, wool, blood...
They can get away with this
because they have
microbe mutualists
4
bacteriome in
aphid
• Special host cells in
body cavity
• Containing 60-90 cells
• bacteria are inside
these cells in vesicles
• each aphid has about
5 million Buchnera
Transmission:
vertical and horizontal
• VERTICAL:
from one generation to the next
• HORIZONTAL :
between individuals that are not parent
and offspring and often not even of the
same species
Vertical
• from one
generation to the
next
• from mother to
offspring
Obligate mutualism
• aphids die without Buchnera
• show this with antibiotics
• Buchnera can’t be cultured outside
aphids – very specific needs
Transmission:
vertical and horizontal
• VERTICAL:
symbiont expect to be completely
faithful to the lineage and co-evolve with
host taxon over time
• HORIZONTAL :
symbiont ‘species’ passed easily among
individuals and species over timewidely distributed
Aphids
• aphids first
appeared about
150-200 million
years ago
• this is a partial
phylogeny
5
Aphids and
Buchnera
Typical of ancient,
obligate lineages
• PERFECT
agreement
between the
trees
• rigorous
vertical
transmission
Go back to Wolbachia
Compare host and symbiont
phylogenies
• What kind of transmission (horizontal or
vertical)?
•
•
•
Hymenoptera: Torymidae: PI
Hemiptera: Delphacidae: CI
Coleoptera: Tenebrionidae: CI
•
Hymenoptera: Trichogrammatidae: PI
•
•
•
•
•
•
•
•
•
•
•
•
Hymenoptera: Encyrtidae: PI
Hymenoptera: Aphelinidae: PI
Hemiptera:Delphacidae: CI
Hymenoptera: Cynipidae: PI
Hymenoptera: Pteromalidae:PI
Lepidoptera: Pyralidae:CI
Hymenoptera: Aphelinidae: PI
Hymenoptera: Eucoilidae: PI
Diptera: Culicidae: CI
Diptera: Drosophilidae: CI
Diptera: Drosophilidae: CI
Crustacea: sowbugs: Fem
Horizontal
Horizontal or Vertical?
• Horizontal transmission required to
explain the distribution of Wolbachia
• Even though there is vertical
transmission, horizontal transmission
must be common over evolutionary
time
• Occasional transfer
between insects
6
Implications of isolation
Mutations will accumulate
=Functions of some genes will be lost
Selection FOR maintaining genes that will
compensate for mutations
Tend to lose genes for which host can take over
function
Selection FOR keeping genes that help host
fitness (making essential amino acids)
Genomes (# of functioning genes) will tend to
decrease in size
Buchnera’s genome
• About 640,000 bp
• Compared to 4.6 million bp in E. coli
A major Buchnera contribution
• essential amino acids
• especially heavy synthesis of
tryptophan
• has 16 copies of tryptophan gene
The Cast
Some aphids have other
symbionts
The Pea Aphid
• parthenogenic
during the summer
(females produce
females= clonal)
7
The secondary symbiont -
Secondary Symbionts
• Secondary symbiont is a younger
associate
• Facultative – not found in all populations
• 3 types identified so far
• What are their functions?
Serratia symbiotica
R
The Parasitoid Wasp
Hamiltonella defensa
Regiella insecticola
T
U
The experiment
• infect SecondarySymbiont-free aphids
with the SS* by injection
• allow wasps to parasitize aphids
uninfected and infected lines
• Are there differences?
wasps lay eggs
in 2nd instar
aphids
* Hamiltonella
What happened to the wasp eggs/larvae?
• parasitism rates
reduced
significantly by 2 of
the 3 SS strains
• (eggs laid in equal
numbers on all
types)
• young aphids
had healthy
wasp larvae
• older larvae
contained many
dead wasp
larvae
8
Noticed variation in resistance
• There are different strains (genetic
differences) of both aphids and
Hamiltonella.
• How do aphid and SS strains interact
with respect to anti-wasp effect?
Different SS strains in same
aphid strain
• all 4 SS strains gave
resistance to aphids
BUT varies from 2982%
• it is something
characteristic of the SS
strain that determines
effect level
Does the same NY1 SS lineage give same
resistance in DIFFERENT aphid lines?
• Resistance varies
9% in first
experiment and 3%
in second
• = same
WHAT causes resistance?
• amplification of genome of Hamiltonella
showed at least two phages (viruses)
associated with it
• the phages encode a gene for cdtB,
cytolethal distending factor, a toxin
• Toxin is the likely agent of wasp death
What does phage do for
Hamiltonella?
• ? allow H. to establish a stable infection
of host cells by distending cell
membranes
• ? protect its aphid host and therefore its
own environment
9
Sap feeding insects
• “fairy land of symbiosis” (Paul Buchner)
• individuals can have up to 6 types of
obligate symbionts, each transmitted in
its own way
• symbionts evolve in parallel with their
host
• pattern of symbiont associates over
time
Genomes of obligate microbial
mutualists decay
genes are damaged over time
numerous chaperonins to make damaged
proteins work
smallest genome of a symbiont contains
only 182 genes
some genes have been transferred to
host genome
what happens when they can’t be
repaired? Stop being useful?
•Sulcia and sap feeders have been associated since the
Permian
•Sulcia has been lost in some lineages
•Baumannia acquired later when xylem feeding began
•Other lineages also acquired additional symbionts
Relationships between microbes
and other organisms have been
going on since the beginning
alliances between
between different forms
of life
The mutualism:
attine ants and their fungi
Fungus growing ants – symbionts outside
• worker ants harvest huge amounts of
leaves and other plant material for their
fungus gardens
• in tropical America, they are the single
most important herbivore
• can be a major pest as they harvest the
plants we nuture in our ‘gardens’
10
Cultivation
the assembly line
• leaves are cut
into smaller and
smaller pieces
• and eventually
‘fed’ to the
fungus
• … and the ants
eat the fungus
Cultivation
• groom the fungus - remove contaminating
spores and parasites
• after about 5 weeks ready to harvest for several
months - easily harvestable knobs
• Protein/fat/carbs are about 24%-2%-27% of dry
weight
• YUM!
Ant-fungal phylogeny
• 50 mya origin of
cultivation by ants
• Higher attines
• Lower attines
•
•
•
•
cut leaves and render them into small pieces
transplant tufts of older fungus to new gardens
space it out so there is room for growth
apply fecal manure
• new queens are produced by parent
colony
• take fungus with them (vertical
transmission)
• mate
• start a new colony and fungus
Higher Attines
• Phylogenies on the
same track
11
Lower Attines
• Free living (black)
• Cultivars (red)
• 3 additional
domestications
• Almost all ants cultivate
more than one type
• New associations being
added
• Similar species, different
cultivars and vice versa
All is not so simple and
peaceful as it seems
• a line of parasitic fungi was discovered
that can quickly kill the true mutualist
• (Escovopsis) - detectable in most
gardens
some researchers noticed white crud on some of
the workers in the fungus gardens
Garden 10 days after infected
with Escovopsis
Actinomycete
bacteria
found mainly on
garden tending
ants
produces antibiotic
that controls
Escovopsis!
Ants-fungiparasitic fungi-bacteria
12